This invention relates generally to compressor wheels or impellers of the general type used commonly with centrifugal compressors in turbochargers, superchargers, and the like. More specifically, this invention relates to an improved centrifugal compressor wheel and its method of manufacture wherein the compressor wheel is designed for substantially prolonged fatigue life.
Centrifugal compressor wheels in general are well known for use in turbochargers, superchargers, and the like wherein the wheel comprises an array of aerodynamically contoured impeller blades supported by a central hub section which is in turn mounted on a rotatable shaft for rotation therewith. In the context of a turbocharger, by way of example, the hub section includes a central axial bore through which the shaft extends, and a nut is fastened over the shaft at the nose end of the wheel to hold the hub section tightly against a shaft shoulder or other diametrically enlarged structure rotatable with the shaft. The shaft thereby rotatably drives the compressor wheel in a direction such that the contoured blades axially draw in air and discharge that air radially outwardly at an elevated pressure level into a chamber of a compressor housing. The pressurized air is then supplied from the chamber to the air intake manifold of a combustion engine for admixture and combustion with fuel, all in a well-known manner.
In recent years, improvements in compressor technology and design have resulted in progressive increases in compressor efficiencies and flow ranges, together with more rapid transient response characteristics. For example, compressor wheels for turbochargers are well known wherein the impeller blades exhibit compound and highly complex curvatures designed for optimum operational effeciency and flow range. Such complex blade shape is most advantageously and economically obtained by a casting process wherein the wheel hub section and blades are integrally formed desirably from a lightweight material, such as aluminum or aluminum alloy chosen for its relatively low rotational inertia for achieving the further advantage of rapid accelerative response during transient operating conditions.
Cast compressor wheels of this general type, however, have a relatively short, finite fatigue life resulting in undesired incidence of fatigue failure during operation. More specifically, when compressor wheel is rotated at operating speeds up to 100,000 rpm or more, cast aluminum, material is subjected to relatively high tensile loading in a radial direction particularly in the hub region of the wheel which must support the radial wheel mass. The impact of this tensile loading can be especially severe when the wheel is operated in a relatively high-speed, rapid speed cycle environment, such as, for example, turbochargers used with earth-moving equipment, front-end loaders, back hoes, and the like. Unfortunately, the hub region of the cast wheel is a site of congreated metallurical imperfections, such as dross, inclusions, and voids, which inherently result from the casting process. The presence of these imperfections in the vicinity of the central bore, which acts as a stress riser, renders the wheel highly susceptible to stress or fatigue fracture in the hub region.
It is known that fatigue failures in compressor wheels can be significantly reduced, or alternately stated, the fatigue life of the compressor wheel can be substantially prolonged by forming the wheel from a noncast material, such as a forged or wrought aluminum or aluminum alloy, thereby avoiding the internal imperfections inherently resulting from a casting process. However, such noncast compressor wheels have not been practical from a cost or manufacturing standpoint primarily due to the complex machining requirements to form the impeller blades with the desired aerodynamic contours.
The present invention overcomes the problems and disadvantages of prior compressor wheels for turbochargers and the like by providing an improved compressor wheel formed from composite materials including cast impeller blades of desired aerodynamic contour and a noncast hub region for improved fatigue life, wherein the cast and noncast materials are secured together in a manner consistent with high production rate manfacturing processes.